<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        ul{
            width: 200px;
            height: 200px;
            background-color: antiquewhite;
            position: relative;
            list-style: none;
        }
        li{
            width: 50px;
            height: 100px;
            position: absolute;
            top: 50%;
            left: 50%;
            transition: all 3s; 
            transform-origin: bottom center;
            background-color: aquamarine;
        }
        ul:hover li{
            transform:rotate(90deg);
            /* transform-origin: center ;会出错！！！！！ */
        }
    </style>
</head>
<body>
    <ul>
        <li></li>
    </ul>
</body>
</html>